This invention relates generally to rotary chucks having jaws for gripping and holding workpieces and rotating together with the workpieces thus held. More particularly, the invention relates to a new balanced chuck in which centrifugal force acting on the jaws (combinations of two jaws comprising a master jaw and a top jaw collectively referred to herein as "jaws") is compensated for.
For reducing machining times and lowering production cost in machine tool operations, it is desirable to increase the rotational speeds of chucks. However, when a chuck is rotated at high speed, centrifugal force arising from the rotation acts on the jaws and decreases their gripping force on the workpiece. For example, the inventor has measured this decrease in gripping force in connection with an increase in the chuck rotational speed with respect to a wedge-type chuck of 160-mm outer diameter generally called a six-inch chuck. As a result, it was found that a rapid decrease in the gripping force begins to occur when a speed of 2,000 rpm is exceeded, and at 4,000 rpm and higher speeds, the gripping force of the chuck jaws is decreased to an extent such that their capability of properly holding the workpiece being machined even becomes doubtful.
Heretofore, numerous proposals relating to countermeasures for solving this problem arising from the action of centrifugal force on chuck jaws have been made. For example, in one type, which may be called the rear-weight type balanced chuck, balance weights are slidably fitted in a manner permitting them to slide in the radial direction in weight-mounting slots in the rear side, with respect to the axial direction, of the chuck body facing the master jaws as disclosed in Japanese Unexamined Utility Model Laid-Open Publication No. 119581/1977, and U.S. Pat. Nos. 2,729,459, 2,657,068, and 4,047,723. In another type, which may be called the side-weight type balanced chuck, balance weights are respectively provided for the master jaws, on one side, with respect to the circumferential direction, or on both sides of each jaw, and are coupled by levers as disclosed in Japanese Unexamined Utility Model Laid-Open Publn. No. 45176/1976, and U.S. Pat. Nos. 2,828,134 and 2,982,558. In still another type, which may be called the crank-weight type balanced chuck, balance weights are mounted on ends of crank levers for actuating respective master jaws as disclosed in Japanese Unexamined Pat. Laid-Open Publn. No. 118181/1976 (corresponding to U.S. Pat. No. 3,984,114) and U.S. Pat. No. 3,370,859.
In each of these balanced chucks of the prior art, centrifugal forces acting on counterbalance weights are utilized to exert forces which are directed inward toward the chuck centerline on the jaws and are substantially equal to the centrifugal forces which are directed in the radial outward direction on the jaws and are thereby balanced, whereby the outward centrifugal forces acting on the jaws are substantially canceled.
The decrease in gripping force of the chuck jaws due to centrifugal force acting thereon will now be considered with respect to a chuck wherein jaws actuated to slide inward toward the chuck axis by a fluid-pressure cylinder-piston mechanism grip a workpiece, and each master jaw has a fitting part fitted in a master jaw mounting slot formed in the chuck body. When centrifugal force arising from rotation of the chuck acts on the jaws, a moment acts on each jaw tending to rotate it away from the chuck axis about the center of the above mentioned fitting part and to cause the front inner end of the jaw to float away from the chuck axis. This is commonly called a "flotation phenomenon" resulting in an elastic deformation of the jaw, which causes a reduction by that much of the jaw gripping force. Since this flotation phenomenon occurs within the range of elastic deformation of each jaw, the jaw is restored to its original state and, theoretically, the gripping force returns to its first-applied initial value when the rotation of the chuck is stopped.
However, in a conventional balanced chuck wherein the force of each balance weight due to centrifugal force is directed towad the chuck axis, the jaws are merely pushed radially inwardly toward the chuck axis at the above mentioned fitting part of the master jaw. For this reason, the jaw is pushed back toward the chuck axis in a state wherein the flotation phenomenon is sustained as it is or, rather, is even augmented, and the centrifugal force acting on the jaw is nullified by increasing the first-applied initial gripping force by the force of the balance weight in order to accomplish gripping force compensation.
Consequently, when the chuck rotation is stopped, only the gripping force corresponding to the decrease in the gripping force due to the flotation phenomenon returns to its original value, and the final gripping force is increased over the first-applied initial gripping force by the force quantity compensated for by the force of the balance weight. As described more fully hereinafter, the inventor has carried out tests on conventional balanced chucks and has found that this increase of the final gripping force over the first-applied initial gripping force ranges from 40 percent to 100 percent or even more.
Thus, the jaws of a conventional balanced chuck grip the workpiece with a tremendous gripping force after completion of a machining operation on the workpiece. Therefore balanced chucks of this character cannot be used for gripping workpieces requiring high machining precision or those having thin walls. The use of these chucks, moreover, is limited because of deformation of the workpiece after machining particularly with respect to die-cast and copper articles and castings of light alloys such as those of aluminum in spite of the fact that these articles require high cutting speeds in machining. If, in considertion of this deformation after machining, a small initial gripping force is first applied to the workpiece, the gripping force during rotation will become too small as a natural consequence, and heavy machining will become impossible.
Accordingly, the balanced chucks of the prior art can be used for holding only workpieces such as bars and articles to undergo rough machining, for example, in the machining of which deformation after the machining operation is not a problem. Consequently, the use of these chucks is limited.